1. Technical Field
The present disclosure relates to a common mode feedback (CMFB) circuit and, more particularly, to a CMFB circuit supporting a dual data rate (DDR) by removing an offset occurring between buses, a programmable gain amplifier (PGA) having the CMFB circuit, and an image sensor having the PGA.
2. Discussion of Related Art
A typical example of an analog integrated circuit is an operational amplifier. Ideally, the operational amplifier has characteristics of infinite gain, infinite input impedance, and zero output impedance. The characteristics of an actual operational amplifier, however, are different from those of an ideal operational amplifier.
For example, in a fully differential amplifier, when a plurality of biases are all fixed without using the CMFB circuit, the range of an output signal of the fully differential amplifier is not secured or the gain is lowered due to a change in power, temperature, or manufacturing process, a change between an input common mode and an output common mode of the fully differential amplifier, or a change in the output common mode caused by noise.
That is, when there is no difference between input signals at a single-ended amplifier, the output of the single-ended amplifier is positioned in the middle of an overall voltage swing range. When the fully differential amplifier has no CMFB circuit, the output of the fully differential amplifier is biased to a level that is not at a middle level between the signals, so that the operation of the fully differential amplifier is restricted.
The CMFB circuit is a negative feedback circuit that detects a common mode voltage of an operational amplifier, compares a reference voltage charged during a reset period with the detected common mode voltage, and drives the detected common mode voltage to be close to the reference voltage, according to a result of the comparison. More specifically, the CMFB circuit is used at an output terminal for setting a common mode of differential output voltages, so that the low-voltage and low-power operations of the operational amplifier are facilitated.
FIG. 1 is a circuit diagram of a conventional CMFB circuit. Referring to FIG. 1, a CMFB circuit 10 includes four capacitors Ce and Ci to forcibly hold a common voltage of output terminals OUTP and OUTN of an operational amplifier and to effectively hold the common voltage at a low voltage, so as to operate as a dynamic CMFB circuit. When the throughput of the data is to be increased by quickly processing the data using a PGA, including the CMFB circuit 10 and an analog-to-digital converter (ADC), one method is to increase the frequency of a system clock signal.
Because there is a structural limit in this method, however, a dual data rate (DDR) structure is used to solve the problem, in which data is output at both a logic high (or rising edge) and a logic low (or falling edge) of the system clock signal.
In a general single data rate (SDR) structure, data of an amplifier is evaluated in a logic high (or rising edge) section of a system clock signal and the amplifier is reset in a logic low (or falling edge) section thereof. A DDR structure does not reset the amplifier to evaluate the data, even in the logic low section, that is, it does not have a reset section.
FIGS. 2A and 2B show the operation of the CMFB circuit of FIG. 1. FIGS. 3A and 3B show the timing diagram in an SDR method and an offset occurring in a conventional DDR method, respectively.
Referring to FIG. 3A, the CMFB circuit compares the output of an operational amplifier (not shown) with a reference voltage in a section of “Reset” of the operational amplifier and refreshes a capacitor Ci with a bias voltage BIAS and a common mode signal line (CML) voltage in a section of “Amplify” of the operation amplifier. For a DDR method, the CMFB circuit 10 is operated as shown in FIGS. 2A and 2B.
Referring to FIG. 2A, when a switching signal Q1B is in a “logic high” state, a pair of capacitors Ci and Ce at each of the left and right sides share charges and generate a CMFB voltage Vcmfb. The CMFB voltage Vcmfb can be expressed by Equation 1.Vcmfb=[{Ci/(Ci+Ce)}×(BIAS−Vcmfb)+{Ci/(Ci+Ce)}×(BIAS−Vcmfb)]/2  [Equation 1]
In an SDR circuit of a general structure, that is, in practicing the method represented in FIG. 3A, when a switching signal Q1 is in a “logic high” state, the operational amplifier is reset, so that valid data is not output from the output terminal of the operational amplifier. On the other hand, in a DDR method, when the switching signal Q1 is in a “logic high” state, the operational amplifier must output valid data, as shown in FIG. 3A.
In this case, however, a capacitance value measured at the output terminal of the operational amplifier varies so that the gain varies accordingly, compared to the case when the switching signal Q1B is in a “logic high” state. Thus, the output of the CMFB circuit 10 varies in the “logic high” section of each of the switching signals Q1 and Q1B so that offset occurs.
As a result, when the CMFB circuit 10 as shown in FIG. 1 is used in a fully differential amplifier used for the DDR structure, the gain of the CMFB circuit varies in the section of “logic high” of each of the switching signals Q1 and Q1B. Accordingly, offset occurs, as shown in FIG. 3B.